The present invention relates to an X-ray generator used in the industrial field, the medical field, etc., and especially, relates to the technique which controls the direction of an electron beam from an electron source arranged inside an X-ray generator.
In the X-ray generator (X-ray tube), an X-ray is generated by accelerating the electron beam generated from a cathode (electron source) which constitutes an electron gun, and by colliding with a target. In a first type of X-ray tube, as shown in FIG. 7, a cathode 102 and a target 106 are arranged in such a way that the outgoing radiation of the X-ray is carried out in parallel with an optical axis O of an electron beam B. FIG. 8 illustrates a second type wherein the cathode 102 and the target 106 are arranged such that the outgoing radiation of the X-ray is carried out in a direction orthogonal to the optical axis O of the electron beam B. The latter type will be explained in detail.
As shown in FIG. 8, an X-ray tube 101 includes the cathode 102 generating the electron beam B; two grids 103, 104 focusing the electron beam B; and a focusing lens 105 focusing the electron beam B. The electron gun consists of the above-mentioned cathode 102, grids 103, 104, and focusing lens 105. The X-ray tube 101 further includes a target 106 generating an X-ray by the collision of the electron beam B from the cathode 102, and an X-ray window 108 is arranged in a vacuum housing 107 which houses the electron gun and the target 106.
When suitable electric potential for the electron gun or the target 106 is applied and the electron beam B is generated from the cathode 102, the electron beam B proceeds in order of the grids 103, 104 and the focusing lens 105 inside the vacuum housing 107, and collides with the target 106. The X-ray is generated by the collision of this electron beam B. When this X-ray is generated, the outgoing radiation of the X-ray is carried out in a direction orthogonal to the optical axis O of the electron beam B, and this outgoing X-ray is taken out from the X-ray window 108. In the case of the former type shown in FIG. 7, the target 106 is arranged to face the cathode 102, and each is arranged in order of the cathode 102, grids 103, 104, focusing lens 105, and target 106, and the outgoing radiation of the X-ray is carried out in parallel with the optical axis O of the electron beam B.
Electric potential of, for example, approximately 100 kV is applied to the target 106. Also, in the case of the latter type shown in FIG. 8, in order to carry out the outgoing radiation of the X-ray in a direction orthogonal to the optical axis O of the electron beam B, the tip portion of the target 106 which is located on the outgoing radiation side of the X-ray is cut aslant. Therefore, an electric field E around the target 106 becomes unsymmetrical to the optical axis O of the electron beam B, and in fact, the electron beam B is bent. Due to the bending of this electron beam B, the electron beam B collides with the target 106 in a location further than the collision location of the ideal electron beam B relative to the X-ray window 108. When the distance between the collision location of the actual electron beam B and the X-ray window 108 (i.e., a generating source of the X-ray) becomes longer than the distance between the ideal generating source of the X-ray and the X-ray window 108, the distance between the generating source of the X-ray and a sample also becomes longer than the distance between the ideal generating source of the X-ray and the sample. Because the distance between the generating source of the X-ray and the sample becomes longer, the magnifying power of the X-ray projection image of the sample declines. Also, because the electron beam B is bent, the ideal optical property of the focusing lens 105 cannot be obtained, so that the diameter of a focus of the electron beam B on the target 106 becomes large, hereby causing the resolution degradation.
In order to solve the above-mentioned problems, the tip of the target 106 is covered by a hood electrode (cylindrical electrode) so as to modify the asymmetry of the electric field E (for example, refer to the U.S. Pat. No. 5,077,771).
However, even if the tip of the target 106 is covered by the hood electrode, because the tip of the target 106 on the outgoing radiation side of the X-ray is cut off but another side of the outgoing radiation side of the X-ray relative to the optical axis of the electron beam is extended to the high voltage supply portion, the asymmetry of the electric field still remains. Consequently, reducing the asymmetry of the electric field further is required.
This invention is made in order to solve the above-mentioned problems, and a purpose of the invention is to provide an X-ray generator which can reduce the asymmetry of the electric field around the target.
Further objects and advantages of the invention will be apparent from the following description of the invention.